This invention relates to a color cathode ray tube (CCRT) having feedback features for an automatic convergence system, and more particularly relates to a photographic process for applying a phosphor pattern to the shadow mask of such a CCRT.
An automatic convergence system has recently been developed for high resolution CCRT displays expected to have application in such demanding fields as computer aided design (CAD) and cartography. See ELECTRONIC PRODUCTS, May 12, 1983, p. 17. Essential to such an auto-convergence system are certain feedback features in the CCRT, which provide information on the location of the scanning electron beams to a computer, which then corrects any misconvergence of the beams. Such feedback features include a phosphor pattern on the back or gun side of the tube's shadow mask, and a window in the side of the tube. When struck by the scanning electron beams, the phosphor pattern emits radiation, some of which is transmitted through the window and detected by an externally placed photomultiplier tube.
The technology for applying the red, green and blue-emitting phosphors to the inside surface of the glass viewing panel of CCRTs is well developed. It is based upon a photolithographic technique in which each of the three phosphor patterns is formed by light exposure of a phosphor-photoresist layer through the shadow mask. Subsequent development and baking of the exposed layer leaves an adherent pattern of phosphor particles on the glass panel.
In contrast, there is no generally accepted technique for applying a phosphor pattern to the mask. Various considerations arise, due to the unique nature of the application. The mask is metallic, bears a protective oxide coating, is relatively fragile and expensive to fabricate. Also, since it is used as the photolighographic "negative" during formation of the phosphor screen on the viewing panel, it becomes "married" to that screened panel. Thus, any subsequent damage to the mask results in rejection not only of the mask but also of the panel. Finally, the mask apertures must be relatively small in size and large in number to produce the desired resolution for high quality display images on the viewing screen. Even partial blocking of the apertures could result in decreased brightness of the image. Any phosphor pattern on the mask which bridges individual apertures, as does the desired phosphor feedback pattern, risks the possibility of blocking of these apertures during pattern application.
In one proposed process, (Ser. No. 496,358, referred to above), the mask is sprayed through a pattern stencil in contact with the mask with a dispersion of fine particle size phosphors in a coating vehicle containing a dispersant and a temporary binder. The sprayed coating is subsequently baked to remove the vehicle and leave an adherent layer of phosphor particles on the mask.
While this process has been used successfully in the manufacture of small numbers of tubes, it has been found difficult to accurately locate and maintain intimate contact between the stencil and the curved surface of the mask. Thus, mislocated and poorly defined phosphor patterns sometimes result. In addition, the need for sufficient phosphor to achieve adequate emission levels for later detection dictates that a relatively thick spray coating be applied. However, as coating thickness increases, phosphor particles accummulate in the area of the mask apertures, decreasing the size, or even blocking such apertures. The phosphor pattern then becomes obvious and distracting to the viewer of the display screen image.